Our research findings to date and pilot studies enable us to formulate a biobehavioral theory of Attention-deficit Hyperactivity Disorder (ADHD). The theory derives from basic animal research on physiological and chemical systems involving the frontal lobe. It invokes two distinct subtypes of ADHD --- underfocused (UF) and overfocused (OF) --- and predicts (a) which test each group will find most difficult, (b) which manipulations will enable improved performance, and (c) in which group a favorable response to stimulant administration is most expected. UF ADHD patients are hypothesized to be handicapped on tasks that are neither intrinsically motivating nor guided by salient (attention-compelling) cues. They will perform better if incentive were provided, task salience enhanced, or stimulant drug administered. This would be expected if orbital frontal cortex were insufficiently activated by limbic (ventral striatal) projections. OF ADHD patients are thought to experience most difficulty in tests requiring flexibility of mental set in relation to changing environmental cues. They should improve as the task becomes routine or amenable to the subject's own habitual strategies --- but not necessarily with stimulants. This would be expected if their nigrostriatal system were overactive, overriding dorsolateral frontal inhibition. We propose a four-phase, five-year research program involving OF, OF and control children aged 6-12 years. ln phase I, we shall supplement our existing test repertoire by designing, instrumenting, and acquiring the necessary normative data on specific procedures that, based on the above behavioral considerations should be differentially sensitive to each pattern of attention deficit. In phase II, we will test the three subject groups and in phase III use those tests which prove most sensitive. We will evaluate predictions about which changes in test parameters will enhance performance of each group. In phase IV, we will examine the interaction between ADHD subtypes, nature of task- relevant manipulations and methylphenidate levels in a laboratory- based double-blind dose-response format. The results should reveal determinants of effective attention in attention-deficient individuals, and provide effective behavioral measures for the attempt to clarify the brain bases of aDHD.